Joint-endoprosthesis and fixation method for the seat thereof

ABSTRACT

The invention relates to a joint endoprosthesis and a method for fixing the seat thereof. The endoprosthesis has a shaft that is anchored in the marrow cavity of a tubular bone, and an ultra source that is placed in or on the shaft and the radiation characteristics of which enable ultrasound waves to reach a gap between the spongiosa of the tubular bone and the surface of the external covering of the shaft. The associated method applies impinging ultrasound waves upon the gap.

BACKGROUND OF THE INVENTION

The invention relates to a joint-endoprosthesis having a shaft that isanchored in the marrow cavity of a tubular bone. The invention furtherrelates to a method of fixing the seat of a joint-endoprosthesis shaftin the marrow cavity of a tubular bone.

Such joint-endoprostheses are known and serve, for instance, to replaceshoulder, arm, hip, knee or foot joints of a human being. All thoseartificial joints have in common that they comprise at least one conicalshaft, which is driven into the marrow cavity of the correspondingtubular bone for fixing the same when the endoprosthesis is inserted. Inmost cases the surface of the external covering of the shaft comprisessupporting ribs bringing about a ratchet-like adherence of the shaft inthe marrow cavity of the tubular bone when the shaft is driven in. Byadditionally feeding small soft bits of spongiosa into the indentationsof the supporting ribs the primary stabilization and the ability of thebone to grow in after the insertion is supported.

It has, however, shown that the feeding of the spongiosa bits not alwaysresults in the desired stabilization of the endoprosthesis shaft. On onehand, this may be due to a non-optimal fit of the shaft in view of therarely ideally formed bone and, on the other hand, to the metabolismprocess being different from patient to patient and the differentosteogenesis connected therewith. Because of this, or also as a resultof inadvertent movements when wearing the already insertedjoint-endoprothesis, luxations of the endoprosthesis with a dislocationby several centimeters may occur. In such cases another operation isoften required, which in most cases takes place in two steps.

An ultrasound device comprising a transportable operating element and anultrasound head is known from U.S. Pat. No. 5,730,705, which can beplaced upon the skin of the patient in the proximity of the region to beimpinged with ultrasound. The propagation of the ultrasound waves andthe formation of shear waves along the shaft of the endoprosthesis areillustrated in FIGS. 4 and 5 thereof. Thus, said known ultrasound deviceserves to impinge the gap between the spongiosa of the tubular bone andthe surface of the external covering of the endoprosthesis shaft withultrasound.

SUMMARY OF THE INVENTION

Reliable fixation of the joint endoprosthesis shaft in the bone is thusa primary concern. It is that problem which this invention addresses,its objective being an improved joint endoprosthesis of the typedescribed above whereby a more stable fixation of the shaft in the boneis attainable.

Given a joint-endoprosthesis of the above-described kind said object issolved in accordance with the invention by providing an ultrasoundsource on or in the shaft, the radiation characteristics of which enablethe ultrasound waves to reach a gap between the spongiosa of the tubularbone and the surface of the external covering of the shaft.

The object underlying the invention is moreover provided in accordancewith the invention by a method of the aforementioned kind, wherein thegap between the spongiosa of the tubular bone and the surface of theexternal covering of the endoprosthesis shaft is impinged withultrasound generated by an ultrasound source disposed on or in theshaft.

In view of the healing of bone fractures the invention makes use of thegained knowledge that the osteocytes required for the healing of afracture multiply by the introduction of ultrasound waves into thefracture gap thereby resulting in a faster, better and more stablehealing of the fracture gap. The background of said effect is that theultrasound stimulates the cell walls of the mesenchyma cells tooscillate, which results in a desired proliferation. In respect of theuse of ultrasound it is essential that it is introduced into thefracture gap, where it impinges and stimulates the spongiosa and thebone marrow, whereas it would be otherwise more or less reflected by thecorticalis of the bone.

The connection of the joint-endoprosthesis shaft with an ultrasoundsource according to the invention and the special radiationcharacteristics thereof directed at the gap between the spongiosa andthe shaft results in the desired stimulation of the cell walls duringthe operation of the ultrasound source and in the proliferation of theosteocytes. The ultrasound source may thereby be disposed externally onthe shaft, or—which most likely is the preferred embodiment—may be partof the shaft or may at least be integrated in the same. A plurality ofultrasound sources may also be distributed over the circumference of thejoint-endoprosthesis shaft, if a regular impingement of the gap betweenthe shaft and the bone can only thereby be obtained.

Advantageous embodiments of the inventions are described in thesubclaims.

Thus it is, for instance, provided that the ultrasound source isarranged such that the shaft serves as an oscillation carrier in theform of a resonance body. It is the goal of said improved embodiment toachieve a regular impingement of the gap between the shaft and the bonewith ultrasound by the shaft uniformly introducing ultrasound into thegap.

Each ultrasound source is preferably part of an ultrasound module withan inherent voltage source so that, for operating the ultrasound source,no voltage has to be supplied from outside, in other words, acorresponding terminal on the patient's body or even an operation arenot necessary.

In order to avoid a large heat generation through the impingement of thegap with high-frequency ultrasound (approximately 1.5 MH) it is providedthat the ultrasound module comprises a multi-vibrator by means of whichthe ultrasound source is pulsed, i.e. operated intermittently. A ratioof 2:8 between working time to non-working time has proved to bepracticable.

It is the goal of the following improved embodiment to keep theultrasound module ready to work for an as long as possible period oftime without the requirement of a new operation and to make itexternally operable by means of a remote control. For this purpose it isprovided that the ultrasound module comprises a switch for switching theultrasound source on/off, and a sensor for the remote-controlledactivation of the switch by means of a signal transmitter. Said signaltransmitter may work in the known fashion likewise on an ultrasoundbasis or on an infrared basis.

In order to make sure what the ultrasound module can be inserted in theshaft of the joint-endoprosthesis as easily as possible prior to theinsertion of the joint-endoprosthesis, for example, for activating thevoltage source beforehand, the ultrasound module is provided with ahousing being accessible from outside by means of a cover plate in theshaft. Said cover plate can be removed by detaching some screws.

Despite a minimization of the current consumption of the ultrasoundmodule it may be necessary to design the voltage source such that it canbe charged from outside.

The improved embodiments of the method according to the inventionprovide a pulsing of the ultrasound such that an impingement lastingabout 200 μsec is followed by a break of about 800 μsec.

BRIEF DESCRIPTION OF DRAWINGS

Two preferred embodiments of the invention in the form of a hipjoint-endoprosthesis will hereinafter be explained in more detail bymeans of a drawing, wherein

FIG. 1 shows a schematic ventral-dorsal vertical profile through aresected femora section with an inserted hip joint-endoprosthesis;

FIG. 2 shows a schematic lateral-medical vertical profile according toFIG. 1;

FIG. 3 shows an illustration according to FIG. 1, however, comprisingseveral ultrasound sources or, respectively, ultrasound modulesdistributed over the circumference of the shaft;

FIG. 4 shows a schematic perspective view of an ultrasound moduleaccording to the detail “X” of FIG. 2; and

FIG. 5 shows a schematic diagram of an ultra sound module.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic ventral-dorsal vertical profile through aresected femora section 4, in the marrow cavity 2 of which a shaft 1 ofa hip joint-endoprosthesis is inserted. The shaft 1 is joined upwardlyin the usual manner by a truncated socket 3 covering the resected femorastump with a collar 20. A gap 6 is illustrated—in an exaggerated sizefor better understanding—between the surface of the external covering 7of the shaft 1 and the inner circumference of the marrow cavity 2. Sucha gap may occur due to a non-optimal fit when, for example, the hipjoint-endoprosthesis is inserted, or due to a luxation with the resultof a dislocation caused by an inadvertent movement or excessive sportiveexercise by the person wearing the endoprosthesis. It is known that thetubular bone 4 of the femora section has a relatively soft porous innerlayer, the spongiosa 8. Moreover, the marrow cavity 2 and thus the gap 6is filled with bone marrow.

An ultrasound module 9 is disposed in the endoprosthesis shaft 1comprising an ultrasound source 5, a voltage source 11, a multi-vibrator13, a switch 15 and a sensor 17 in a housing 12 as essential elementscompare FIG. 5). The housing 12 is sealed with an ultrasound-permeablecover plate 14 on the surface of the external covering 7 of the shaft 1.During the operation of the ultrasound module the radiationcharacteristics of the ultrasound source 5 enable ultrasound waves toreach the gap 6 between the spongiosa 8 of the tubular bone 4 and thesurface of the external covering 7 of the shaft 1 so as stimulate thespongiosa 8 and the (non-illustrated) bone marrow to proliferateosteocytes.

The switching on and off of the ultrasound source 5 is preferablyeffected by means of a remote control in the form of a signaltransmitter 19, which emits infrared or ultrasound radiation 18 via thesensor 17 for activating the switch 15.

FIG. 2 shows a schematic lateral-media vertical profile through thetubular bone 4 according to FIG. 1 enabling the view onto the coverplate 14 of the ultrasound module 9.

With the goal to supply the gap 6 around the shaft 1 with ultrasoundenergy as uniformly as possible, a shaft of the joint-endoprosthesis is,according to a second embodiment, provided, having a plurality ofultrasound sources 5′, 5″, 5″′ . . . distributed over the circumferenceof shaft 1. Each ultrasound source again forms part of an ultrasoundmodule 9′, 9″, 9′″ . . . , all of which may be switched on or off in theabove-explained manner.

FIG. 4 shows a perspective illustration of an ultrasound module 9 havinga housing 12 and a front plate 21 attached on the housing 12 by means ofscrews 16. Said front plate 21 may be identical to the cover plate 14 inthe mounted state of the ultrasound module 9.

FIG. 5 shows a schematic diagram of an ultrasound module 9. As wasalready described above, the module comprises a voltage source 11, amulti-vibrator 13, an ultrasound source 5, a switch 15 as well as asensor 17, all of which are accommodated in the housing 12. In view ofthe cover plate 14 or, respectively, in view of the endoprosthesis shaft1 the ultrasound source 5 is thereby arranged such that an optimumexposure to sonic waves of the gap 6 with the spongiosa 8 containedtherein and the bone marrow is secured.

The method according to the invention is once more briefly explained bymeans of FIG. 1: By using the knowledge that the formation ofosteocytes, which are, for instance, necessary for the healing of a bonefracture, is supported by the impingement of the mesenchyma cells withultrasound, the method according to the invention provides, for makingthe seat of a joint-endoprosthesis shaft 1 in the marrow cavity 2 of atubular bone 4 more stable, that the gap 6 between the spongiosa 8 ofthe tubular bone 4 and the surface of the external covering 7 of theshaft 1 is impinged with ultrasound by means of an ultrasound source 5.For avoiding an excessive generation of heat in the gap 6 theimpingement with ultrasound is pulsed so that after an impingementlasting about 200 μsec a break of about 800 μsec occurs. A usual outputof the ultrasound source 5 is about 30 mW with a sound frequency ofabout 1.5 MHz.

What is claimed is:
 1. A joint-endoprosthesis comprising a shaft (1)having an external covering (7) and being anchored in the marrow cavity(2) of a tubular bone (4) having spongiosa (8) an ultrasound source (5)on or in the shaft (1), said ultrasound source having radiationcharacteristics which enable ultrasound waves to reach a gap (6) betweenthe spongiosa (8) of the tubular bone (4) and the external covering (7)of the shaft (1).
 2. The joint-endoprosthesis according to claim 1,wherein the ultrasound (5) is arranged such that the shaft (1) serves asan oscillation carrier.
 3. The joint-endoprosthesis according to claim1, wherein a plurality of ultrasound sources (5′, 5″, 5″′ . . . ) aredistributed around the shaft (1) or disposed in or on the shaft (1). 4.The joint-endoprosthesis according to one of the claims 1 to 3, whereineach ultrasound source (5′, 5″, 5′″ . . . ) is part of an ultrasoundmodule (9) having an inherent voltage source (11).
 5. Thejoint-endoprosthesis according to claim 4, wherein the ultrasound module(9) comprises a multi-vibrator (13) with which the ultrasound source (5)is pulsed.
 6. The joint-endoprosthesis according to claim 4, wherein theultrasound module (9) comprises a switch (15) for switching on/off theultrasound source (5) and a sensor (17) for the remote-controlledactivation of the switch (15) by means of a signal transmitter.
 7. Thejoint-endoprosthesis according to claim 4, wherein the ultrasound module(9) comprises a housing (12) accessible from outside through a coverplate (14) in the shaft (1).
 8. The joint-endoprosthesis according toclaim 4, wherein the voltage source (11) is charged from outside.
 9. Amethod of strengthening the retention of a joint endoprosthesis as inclaim 1, including the step of impinging said ultrasound waves upon saidgap (6).
 10. The method according to claim 9, wherein the impingementwith ultrasound is pulsed.
 11. The method according to claim 10, whereinan impingement lasting about 200 μsec is followed by a break of about800 μsec.